Ceramics are being widely considered for replacement of metals and polymers in many applications. Their corrosion resistance, small coefficients of thermal expansion, light weight, low cost, and in many cases, high strength at high temperature, make them very attractive in microelectronics, structural, and biotechnology areas.
Despite their desirable properties, ceramics are quite brittle and difficult to produce. Current processes involve preparations of small particles, transferral to a mold while suspended in a fluid vehicle, removal of the fluid, and subsequent sintering at high temperature to densify the ceramic. Presently, only limited methods are available to control particle size, composition, and surface properties. Nonuniformities in particle size make sintering difficult and result in internal flaws which weaken the final ceramic body. Ceramics with structural strengths in excess of those now in existence can be developed if improved ceramic powder technologies can be devised to produce ceramics characterized by fewer and smaller defects.
The manufacture of ceramic bodies reaching theoretical density requires sintering of a particulate compact or green body. Any pores remaining will act as stress concentrators and will promulgate premature fracture of the ceramic body. Pores much larger than the original particle size will form between particle aggregates in the green body. These large pores are difficult to remove and will frequently grow at the expense of smaller pores between well-packed particles. Only if the number of particles that surround a void (coordination number) is smaller than a critical value will these voids shrink under reasonable hydrostatic sintering pressures
It is suspected that the primary source of the defects and the low bulk density is the presence of a widely scattered particle size distribution. Work with analogous polystyrene "monodisperse" spheres indicates that line defects and vacancies invariably nucleate at particles much larger or smaller than the average. Line defects are especially serious since they can disrupt packing over tens of particle diameters. Rigorous removal of particles at extreme ends of the size distribution would be a first step toward improving ordered green body packing.
The usual method of preparing fine particle oxide salts from oxalates, acetates, and carbonates is to thermally decompose, pyrolyze, or hydrolyze them to their oxides. A recently developed method based on the controlled precipitation of oxides from solutions of appropriate water reactive metal alkoxides has been the most successful to date in producing unagglomerated spherical particles with a diameter of approximately 0.2.mu.. Unfortunately, the size dispersion of the particles is no better than .+-.10%.
Many of today's newest and most promising ceramics, such as silicon nitride (Si.sub.3 N.sub.4) or silicon carbide (SiC) are not processible by controlled hydrolysis of alkoxide solutions. For these ceramics, other methods of particle formation are required. One of the most effective methods for the production of fine, ca. 0.05--.mu., particles of SiC involves laser-induced reaction of SiH.sub.4 and CH.sub.4. Conveniently, the CO.sub.2 laser employed emits most strongly at 10.6.mu. a strong vibrational absorption of the reactants. Using focused laser radiation, temperatures of 1500.degree. can be developed in 10.sup.-4 sec to provide complete conversion to an unagglomerated ceramic powder which compacts well. The green bodies sinter to dense ceramic bodies with the usual compliment of voids, most probably because of the dispersion in particle size. More recently, mixed SiC-Si.sub.3 N.sub.4 powders have been produced by the same process using [CH.sub.3).sub.3 Si].sub.2 NH gas as a ceramic precursor. Unfortunately, incomplete reaction in the gas phase precluded complete conversion to ceramic, and agglomerates formed through the cohesion of "sticky" polymer layers that remained on the ceramic particle surfaces. Metal oxides can also be produced by simple resistance heating of a tube through which is passed a metal alkoxide.
Another powerful technique for the production of more controlled particle sizes is through an aerosol. Typically, alkoxide vapor is generated, condensed, and reacted with a counterflowing, wet, inert gas stream. Alternatively, the aerosol can be made by any number of nebulizing techniques such as a piezoelectric excitation of periodic surface instabilities in a layer of liquid or across a stream of liquid.
A difficulty with all the above discussed methods of particle production is that the particles are still not truly monosized when synthesized. Additionally, there is no generic way to produce monosize particles of nonoxide ceramics. It has been found that the present method for particle growth can employ a polymethylmethoacrylate latex of very narrow size distribution as a seed phase for the incorporation of preceramic organometallic. These particle suspensions are then the raw material for introduction into an aerosol dispersor which will separate the particles in a gas stream and permit them to be converted to ceramic. Swelling of monosized latex particles of small size (0.1-0.5.mu.) with monomer and subsequent polymerization permits the formation of large 1-10.mu. monosize particles (.+-.1% variation in diameter). Previous research has been conducted on the swelling of polystyrene particles with organic compounds. These previous techniques require pre-swelling treatment and were further improved to a one step method through the emulsions. This work has involved the swelling of organic particles with organic reagents only. The process described herein is designed to produce ceramic not organic particles.